Pocket radiation meter



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Patented Apr. 7, 1953 POCKET RADIATION METER Francis R. Shonka, Riverside, Ill., assigner to the United States of America as represented by the United States Atomic Energy Commission Application May 28, 1951, Serial No. 228,718

7 Claims.

This invention relates to a radiation measuring device; and more particularly it relates to an improved electrometer and an improved charging switch which are particularly adaptable for use in radiation measuring devices of the type generally referred to as dosimeters.

The health of personnel employed in handling radioactive materials may be jeopardized if pre- `cautions are not taken to see that each individual Workman receives only a small enough amount of radiation over a period of time that no permanent damage is done to his body. One of the best ways to ascertain the amount of radiation to which an individual workman has been subjected is to have the workman carry a radiation dosimeter which will give a measurement of the radiation to which it has been subjected while being carried upon the workman, and thus give a measurement of the radiation incident upon the workman. As the number of people engaged in handling radioactive materials has increased, the demand for dosimeters suitable for this purpose has also increased, and one of the objects of the present invention is to provide an electrometer which is suitable for use in a dosimeter and which can be easily calibrated, thus lending itself readily to mass production techniques. To accomplish this result, the present device utilizes an 'adjustable electrometer which may be used with a preformed scale, rather than requiring a separately constructed and calibrated scale to be used with each individual electrometer.

`Previous attempts to mass produce dosimeters have indicated that much custom work in assembling a switch has had to be tolerated. A switch -is necessary in a dosimeter, since it functions to isolate at least one of the electrodes within the dosimeter from the terminals on the exterior surface of the instrument, and thus eleminates discharging of the dosimeter as a result of leakage currents flowing between the terminals on the exterior surface of the instrument. 1t is to be remembered, that these leakage currents are of much greater magnitude than would be expected from the composition cf the materials of the exterior surface due to the fact that moisture from the human body collects upon the surface of the dosimeter and greatly reduces the leakage resistance thereof. The switch in a dosimeter must not only perform the usual functions of a switch, but it must also perform these functions without piercing the gas-tight seal between the interior of the dosimeter and .the ambient atmosphere. Generally, gases other :than air at atmospheric pressure are used in the desmeter ionization chamber, and these gases will escape or be contaminated if theambient atmosphere is permitted to seep into the dosimeter. In the past, mass production attempts to manufacture dosimeters have been greatly hampered by the necessity of doing custom work to assure the dosimeter a gastight seal which is not broken by actuating the switch. Hence, another object of this invention is to provide a dosimeter switch which may be readily assembled into the dosimeter and Which provides a certain gas-tight seal at all times.

Dust within the ionization chamber will also result in a partial discharge of the chamber, and hence during assembly of the dosimeters that have been constructed in the past much effort has been expended to avoid dust. However, no amount of effort in assembly will completely eliminate dust from the chamber, particularly in view of the fact that jarring of the dosimeter when in use causes small particles to be set free from the walls of the ionization chamber. Thus, it is another object of the invention to provide a means for nullifying the effects of dust, so that the efforts to eliminate dust which have been expended in assembling dosimeters in the past may be greatly curtailed.

Other objects and advantages of the present invention will become apparent upon a further reading of the following specication and claims. The invention may be more clearly understood by reading the specication in conjunction with the drawings in which:

Figure 1 is a longitudinal sectional view of a dosimeter embodying the present invention;

Figure 2 is an enlarged transverse sectional view taken along line 2--2 of Figure 1 illustrating in particular the construction of the switch portion of the dosimeter;

Figure 3 is an enlarged transverse sectional view taken along line 3 3 of Figure 1;

Figure 4 is a View of a device for Calibrating the electrometer, partly in section and partly schematic; and

Figure 5 is a view of the charging device for the dosimeter, also partly in section and partly schematic.

The dosimeter generally consists of an optical system l0, an ionization chamber l2 including an electrometer i6, and a charging switch I4, as shown in Figure l. The optical system ll) and the ionization chamber l2 are mounted in a cylindrical electrically conducting housing I8, which may be of brass or aluminum, for example. The charging switch ill is mounted in a supporting tube 2c which has an inner diameter approximately equal to the outer diameter of the cylindrical housing, so that a portion of the housing I8 is snugly fitted within the tube 2li forming a single sealed assembly. The junction between the tube 2i) and the housing IS has been made Without a ridge appearing on the outer surface of the assembly by making the outer diameter of the tube 2i) equal to the outer diameter of the housing I8, and cutting away the end portion 22 of the housing I8 to a diameter approximately equal to the inner diameter of the tube 20. The end portion 22 of the housing I8 nts snugly into the tube 2i! and forms a gas-tight seal in this manner. The charging switch I4 is cylindrical in shape with a diameter approximately equal to the inner diameter of the tube 2Q, and the charging switch It is snugly mounted into the open end of the tube 2li sea-ling the in! terior of the assembly from the ambient atmosphere. While the charging switch I l and the housing I8 may be inserted into the tube 20 snugly enough to form gas-tight seals, it is generally desirable to use a sealing compound between these elements and the tube 20. The tube 2) may readily be fashioned from one of various forms of plastic, in which case there are many good plastic and plastic-to-metal sealing compounds known in the art. A cap 2li is removably mounted about the end of the tube 2@ adjacent to the charging switch I4 to protect the switch terminal from mechanical damage. The cap 2Q completes the dosimeter.

The optical system iii focuses upon the elecu trometer I6, and may be any one of the many well known systems. The optical system It shown in Figure l consists of two Ramsden eyepieces which are cascaded in order to give sufcient magnication with lenses having less curvature than would otherwise be required. It also results in an erect image, while the single Ramsden eyepiece results in an inverted image, but an inverted image could equally well be used in the present optical system. Four plano-convex lenses 28a, 25h, 2te and 26d are used. The curved sides of lenses 25a and 26h face each other, as do the curved sides of lenses 26o and 26d. Since the Ramsden eyepiece has a focal plane preceding the eld lens, here lens 2Gb, a scale 28 is mounted in the housing I8 on this plane. In this manner, the scale 28 appears superimposed upon the electrometer IS. The lenses 2E and the scale 23 are cemented. into the housing I8, and Stoppers 2? placed between the lenses and the scale accurately maintain proper distances. A ridge 16 on the end of the housing I adjacent to the ocular lens 26a prevents the Ylenses 26 from sliding out of this end of the housing I3.

The electrometer I includes a supporting wire 30 which is mounted upon a transparent disc 32. The disc 32 is transversely mounted in the end portion 22 of the housing I8. The disc 32 may be of glass or other transparent materials which may be readily sealed to the housing I8 by any of the well known glass-to-metal sealing means. An aperture 34 through a portion of the disc 32 aids in the transmission of light through the dosimeter.

The electrometer IB is provided with a :liber 33 which is mounted upon a U-shaped fiber support 36V, and also assumes this shape. The fiber 38 may be a quartz fiber. The ber support 36 .is

Yprovided with a small bore it on the end opposite the fiber 38 into which the supporting wire 3B is wedged and secured. The entire electrometer I6 is mounted slightly on the center axis of the housing I8, and forms the inner electrode of the ionization chamber I2. The outer electrode is the inner surfaces of the housing I8 and the tube 26, together with the coating thereon to be described later.

A n d2 is cut from the end portion 22 of the housing I8 on the surface contiguous to the fiber 38. The iin 42 is mounted on the opposite side of the fiber with respect to the fiber support 3B. The iin i2 is relatively narrow, and many times its width in length. However, the iin 42 is not long enough to contact the fiber 38, even in their closest positions.

A mixture of graphite and grease is used as a coating All on the inner surface of the housing I8 and on the inner surface of the supporting tube 2B between the charging switch Iii and the optical system It. 1t forms a surface having relatively good electrical conductivity which will catch dust and dirt particles in the ionization chamber I2, and prevent them from having an injurious eiect upon the operation of the ionization chamber.

'I'he switch charging mechanism I4 is sealed into the open end of the supporting tube 25J, and consists of a switch housing 4t which is cylindrical in shape having an outside diameter approximately equal to the inside diameter of the supporting tube 2B, a contact rod 48 movably disposed alcng the axis of the switch housing 46, and an armature 5D in the shape of a rectangular bar attached to the contract rod A8 approximately at its center. The switch housing 46 consists of a cylindrical member 52 which is snugly fitted into the supporting tube 20 of the dosimeter, a disc 54 attached to the inner end of the cylindrical member and is provided with an aperture 56 at its center having a diameter suiiicient to permit motion of the contact rod i8 therethrough, a second disc 58 attached to the open end of the cylindrical member 52 having an orifice 60 at its center which is sealed to a sleeve terminal 62, and a pair of disc-shaped keys 64 mounted within the cylindrical member 52, one of the disc-shaped keys being attached to the inner surface of the disc 5l! and the other key being attached to the inner surface of disc 58. The sleeve terminal t2 is provided with a central bore 63 into which the one end of the contact rod 4S is slidably mounted. All of the parts of the switch housing 4S are constructed of transparent materials, for example methyl methacrylate resin, in order to permit the passage of light into the interior of the dosimeter so that the position of the fiber 38 of the electrometer I6 may be Viewed through the optical system I0. Slots 66 in the ring-like keys E4 face the armature 53 and are slightly wider than the depth of the armature 50, so that the armature may be inserted therein. The slots 56 are oriented normal to each other also, the ringlike keys 64 are spaced from each other along the central axis by a distance which is slightly greater than the height of the armature, so that the armature may be rotated between the ringlike keys 64. A spring 6l is spiraled about the contact rod 48 and compressed between the armature 50 and the disc 54 adjacent to the ion chamber I2, thus forcing the armature 5G against the ring-like key 64 farthest from the ionization chamber I2. The remote end of the electrometer supporting wire 3] from the electrometer IS is bent into a position on the axis of the tube 2d, and forms a contact element G8 for the charging switch I4. The contact element 68 is directly in line with the contact rod 48, and motion of the contact rod 48 against the spring 81 would bring the contact rod 48 into contact with the contact element S6 were it not for the stop provided by the intermediate ring-like key 64. However, the slot 66 in the ring-like key G4 intermediate the armature 58 and the contact element 88 is of suficient depth to permit motion of the contact rod 48 into contact with the Contact element 63 when the armature 5c registers with the key 6,4. The armature 58 may be constructed of any ferromagnetic substance, such as iron for example.

A cap 24 constructed of transparent material, such as transparent plastic, is slidably attached to the end of the supporting tube 2Q, and covers the sleeve terminal 62.

The dosimeter may be very readily assembled. The ocular lens 28a is slid into the cylindrical housing I8 from the electrometer end of the housing I8, it being noted that the ridge 18 is provided to retain the lens 26a within the housing I8. A stopper 21 of suitable length, as determined by the focal lengths of lenses 26a and 22h, is then slid into the housing IS into contact with the lens 28d. Lens 2612 may then be slid into place, followed by a second stopper 21. The scale 28 may then be slid into position against the stopper 21 and secured to the housing I8 by cement. It is also preferable that the lenses 2E be cemented into place. A long stopper is then slid into the housing I8y then the lens 28o, the stopper 21, and the lens 26d. A i-lnal thin stopper 21 is then slid into the housing I8 and cemented securely to the housing in order to make certain that the optical system IIJ will be rigidly secured in place. The electrometer I6 is previously assembled on the transparent disc 32, and the transparent disc 32 may now be cemented into the end portion 22 of the housing I8, thus mounting the electrometer in place.

The next step is to calibrate the electrometer i6. Figure 4 shows the device for accomplishing this calibration. A charging ring 12 is placed around the end portion 22 of the housing I8. It is provided with a threaded wing screw 14 which is threaded into the charging ring 12l the screw 14 being positioned immediately adjacent to the iin 42 in the end portion 22 of the housing i8 by positioning the ring 12 at the proper position. The electrometer I6 is connected to a source of charge, such as a battery 16, through a switch 18 and a potentiometer 80, which is connected across the battery 1S. A voltmeter 82 is connected between the adjustable tap 8I ci the potentiometer 80 and one of the terminals of the battery 16. That terminal is then connected to the end portion 22 of the housing i8. The switch 18 is then closed, causing the ber 3S to be deflected outwardly into the dotted position 0i Figure 4. The potential applied to the electrcmeter is then adjusted to a predetermined calibration value by adjusting the tap 8l of the potentiometer 80 until the voltmeter 82 reads the desired value which is usually the voltage resulting in full scale deiiection which will be the zero indication reading. The fiber 38 may then be deflected further until the proper scale reading is obtained by bending the n 42 toward the ber 33 by screwing the Wing screw 14 into the charging ring 12. In this manner, fibers having slightly diierent physical characteristics may be placed in electrometers constructed by mass production methods.

The charging switch I4 may then be inserted into the supporting tube 28, and the graphite and grease coating 44 applied to the inner sur- 6 face of the supporting tube 20 and to the inner surface of the end portion 22 of the housing I8. The end portion 22 of the housing I8 may then be inserted into the supporting tube 20. Since the supporting tube 20 is constructed of plastic and will be somewhat stretched, the housing I8 may be wedged into the tube 20. A cement may be used to assure a tight fit between the housing I8 and the tube 20, if necessary. When the housing I8 has been thoroughly inserted into the tube 20, the contact element 68 is sufciently close to the contact rod 48 that contact may be made between these elements when the rod 48 is moved against the spring 61 and the armature 5I) is in register with the slot 66 of the key 64 in the path of the armature 50. It remains then only to place the cap 24 onto the end of the supporting tube 20 to complete the dosimeter.

The cap 24 is removed from the dosimeter when the dosimeter is to be charged. A charging socket such as that shown in Figure 5 may be used to charge the dosiineter. yThe electrical circuit is essentially the same circuit as appears in Figure 4, and the same numbers have been assigned to the elements of the circuit in Figure 5 that have been assigned to their counterparts in Figure 4. The charging socket, which is adapted to receive the switch-end of the dosimeter, has a tubular element 98 with an inner diameter slightly greater than the outer diameter of the supporting tube 28 of the dosimeter. The one end of the tubular element is open, and the other end is closed. An electrical contact B2 is mounted on the axis of the closed end of the tubular element 90, and extends through the end of the tubular element 90. A contact ring 94 is mounted in a recess 95 on the inner surface of the open end of the tubular element 90. The tubular element S0 is provided with a pair of orices 9d which extend through the walls of the tubular member 88 on a common axis which also intersects the central axis of the tubular element Si! at a point from the closed end of the element $8 approximately equal to the distance from the end of the sleeve terminal 62 to the stationary contact element 58 in the dosimeter. Poles 98 extend into the orices 95 to the inner surface of the tubular element 98 and are attached to a magnetic yoke IBS. A magnetic coil |02 surrounds the center portion of the magnetic yoke I8il and is connected to a battery I04 through a switch 06.

The dosimeter, with the cap 24 removed, is inserted into the open end of the tubular element 98, and in this manner the end oi the sleeve terminal E2 of the dosimeter is placed in contact with the electrical contact 52 at the closed end or the tubular element S8 in the charging socket. The conducting housing i 8 also in contact with the contact ring 84 at the mouth of the tubular element 9G. Tap 8i or the potentiometer 88 is adjusted until the voltrneter 22 indicates that the desired charging voltage will be applied to the terminals of the dosimeter. Usually, this Voltage is the potential which will give a full scale deiiection of the electrometer i Switch 18 is then closed applying the charging potential to the terminals of the dosimeter. In order to charge the dosimeter, however, it is necessary that the charging switch I4 within the dosimeter be also closed. Hence, switch tdt is closed energizing the coil Iii-2 and establishing a magnetic iield through the charging switch Ill of the dosimeter, since it has been piaced between the magnetic poles 98. The dosimeter is then rotated within the tubular element 9D, usually by hand, but a mechanical device could readily be devised for this purpose. Since the magnetic armature will align itself with the strongest portion of the magnetic eld, the armature 5D will be drawn toward the contact element 68, and when the armature is rotated to register with the slot 66 in the key 65. contiguous to the ionization chamber I2, which will occur twice for each revolution of the dosimeter within the tubular element 98, the contact rod i8 will be forced into contact with the contact element E8. In this manner, the charging voltage measured by the voltmeter 82 will be applied to the ionization chamber l2 of the dosimeter, and the capacity of the chamber I2 will be charged to the potential indicated by the voltmeter 32.

The dosimeter is then removed from the tubular element 9B with a rotational motion, so that the armature 59 will be rotated out of register with the slot 66 in the key Bti contiguous to the ionization chamber i2, and will be rotated to register with the slot 66 of the key 6@ remote from the ionization chamber I2. Since the spring El will maintain a pressure upon the armature in the latter slot G6, there is little likelihood of the armature 59 rotating 9i) degrees to register with the other slot and to permit the contact rod e8 to contact the contact element 68.

In a particular embodiment of this invention, the maximum voltage to which the ionization chamber was charged was 180 volts, and the operating voltage range for the ionization chamber extended down to approximately 120 volts. Maximum bending of the n i2 toward the electrometer I6 had about the same eiect as increasing the voltage to which the ionization chamber was charged by approximately 15 volts.

From the foregoing description, it is clear that a dosimeter may be constructed using the teachings of this invention which will be adapted to production on a mass scale. Many other embodiments and variations in the present invention may be made by the man skilled in the art, all within the intended scope and spirit of the invention.

What is claimed is:

1. A device for measuring particles and radiations comprising, in combination, a gas-tight housing, an ionization chamber mounted within the housing, and a switch mounted within the housing including a pair of contacts, one of said contacts being connected to one of the electrodes of the ionization chamber and the other contact being connected to the exterior of the housing, one of said switch contacts being stationary and the other contact being a rod slidably mounted upon an axis intersecting the stationary Contact, an armature aixed to the Contact rod, and a pair of slotted members mounted with slots at right angles to each other and normal to the axis of the contact rod, the slots in said members being of suiicient size to accommodate the armature, and the members being aixed to the housing on opposite sides of the armature and spaced from each other by a distance greater than the height of the armature measured along the axis of the contact rod, the distance between the bottom of the slot in the slotted member mounted between the armature and the stationary contact being less than the length of the portion of the contact rod between the armature and the end thereof confronting the stationary contact, and the distance between the top of said slot and said stationary contact being greater than the length of said portion of the contact rod.

2. A device for measuring particles and radiations comprising the elements of claim 1 in combination with a tube, a pair of sheet-like elements mounting the switch to the housing, the tube aiixed to the housing, and the sheet-like elements being disposed transversely across the open ends of the tube, the slotted members, armature and contact rod being disposed within the tube between the elements, each of said elements being provided with an orifice on the axis of the contact rod, the orifice in one of said elements confronting the stationary contact, a portion of the Contact rod being slidably disposed in said oriiice, and a tubular terminal of electrical conducting material sealed into the oriiice in the other of said elements, said tubular terminal having an inner diameter slightly greater than the diameter of the contact rod and being sealed on the end remote from the armature, one end of the contact rod being slidably disposed within said tubular terminal to make electrical contact therewith.

3. A device for measuring particles and radiations comprising the elements of claim 2 in combination with means to charge the ionization chamber including a charging socket adapted to receive the end of the housing adjacent to the switch and to permit rotation of the housing therein, electrical contact means to contact the sealed end of the tubular terminal of the switch which is adapted to be connected to one of the electrodes of the ionization chamber and to contact the other electrode of the ionization chamber, a source of charge connected to said electrical contact means, and means mounted adjacent to the charging socket to establish a magnetic eld through the interior of the socket, whereby the magnetic eld may be used to orient the armature into register with the slot of the slotted member between the armature and the stationary contact of the switch and to move the contact rod into contact with the stationary contact.

4. A device for measuring particles and radiations comprising the elements of claim 1 in combination with means exterior to the housing to establish a magnetic eld to orientate the armature and to move the contact rod into contact with the contact element.

5. A device for measuring particles and radiations comprising, in combination, and ionization chamber having a plurality of electrodes and an ionizing medium therebetween, and a coating including a mixture of graphite and grease on the inner surface of the ionization chamber.

6. A device for measuring particles and radiations comprising, in combination, a plurality of electrodes, an ionizing medium therebetween, means to seal the ionizing medium from the ambient atmosphere, and a coating of graphite and grease on at least a portion of the surfaces of the combination adjacent to the ionizing medium.

7. A device for measuring particles and radiation comprising, in combination, a gas-tight housing, an ionization chamber mounted within the housing and a switch mounted within the housing including a pair of switch contacts, one of said contacts being aiiixed to the housing and connected to one of the electrodes of the ionization chamber, and the other contact being slidable within the housing and connected to the exterior of the housing, said housing being provided with at least two spaced slots parallel to the path of the slidable contact and angularly displaced from each other relative to the path of 9 the slidable contact and a. recess between said slots, said slidable contact being provided with a. protruding portiouadapted to slide within the slots and rotate in the recess between the slots and the housing.

FRANCIS R. SHONKA.

REFERENCES CITED The following references are of record in the file of this patent:

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